This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-394464 filed on Dec. 26, 2001.
1. Field of the Invention
The present invention relates to a method for assembling a motor having an armature, which is rotated through engagement of brushes with a commutator of the armature, and also relates to a brush holding device and a motor having the same.
2. Description of Related Art
In general, a direct current motor includes brushes and a commutator to mechanically commutate externally supplied direct current and to supply it to an armature. Each brush is supported by a corresponding brush holding device. The brush is assembled such that the brush slidably engages the commutator secured to the armature. During manufacturing, each brush holding device is assembled to a housing of the motor before installation of the armature. In the brush holding device, the brush is temporarily held in a retracted position in a brush holder against urging force of a spring before installation of the armature in a housing of the motor. Then, when the armature is installed in a predetermined position in the housing of the motor, the brush is released from the retracted position, so that the temporarily held brush is urged against the commutator to slidably engage the commutator.
As a first example, the following brush holding device has been proposed to achieve the above assembling operation. Each of brushes, which are urged by springs, is engaged with an outer peripheral surface of a ring member and is temporarily held in a retracted position in a brush holder of a corresponding brush holding device. After each brush holding device is assembled to a housing of the motor, the armature is installed to a predetermined position in the housing of the motor. At this time, the ring member is pushed downward by an end surface of the commutator secured to the distal end side of the armature. Thus, each brush, which has been engaged with the outer peripheral surface of the ring member, is now engaged with the commutator instead of engaging with the ring member.
As a second example, the following brush holding device has been proposed to achieve the above assembling operation. A brush holder of each brush holding device, which holds a corresponding brush in a manner that allows protrusion and retraction of the brush relative to the brush holder, is provided with a tongue-shaped hook that extends toward a commutator side. The hook restrains movement of the brush, which is urged toward the commutator side by a spring, to temporarily hold the brush in a retracted position in the brush holder. When the armature is installed in a predetermined position in a housing of the motor, the tongue-shaped hook is bent, so that the temporarily held brush is released. Thus, the brush, which has been restrained to move toward the commutator side (i.e., restrained to protrude from the brush holder) by the hook, now protrudes from the brush holder and engages the commutator.
In the first example, the additional component, i.e., the ring member, is used to temporarily hold the brush at the time of assembly, resulting in an increase in a manufacturing cost. Furthermore, the motor needs to have a space for accommodating the ring member, which is used only for temporarily holding the brushes.
In the second example, similar to the first example, the temporarily held brush in the brush holder is released by the insertion of the armature. Thus, the freedom of the design has been limited in terms of assembling timing of the armature, a positional relationship between each brush and the commutator and the like. Furthermore, when the brushes do not simultaneously engage the commutator, the shaft of the armature can be displaced by the urging force, which radially urges the corresponding brush. Thus, during assembly of the armature, it could disturb the insertion of the shaft distal end of the armature into a corresponding shaft receiving hole. Furthermore, the armature moves continuously after a moment of releasing the temporarily held brushes. In such a case, the released brushes could form an axially extending scratch on a surface of the distal end side of the commutator of the armature. Since the commutator is engaged with the brushes in a circumferential direction during operation of the motor, the axially extending scratch can disturb smooth commutation.
The present invention addresses the above disadvantage. Thus, it is an objective of the present invention to provide a method for assembling a motor achieving engagement of each brush to a commutator without using insertion movement of an armature. It is another objective of the present invention to provided a brush holding device, which allows engagement of a brush with a commutator without using insertion movement of an armature at assembly of the armature and also allows a reduction in a manufacturing cost of the brush holding device. It is another objective of the present invention to provide a motor having such a brush holding device.
To achieve the objectives of the present invention, there is provided a method for assembling a motor. In the method, a brush is temporarily held in a first position where the brush is retracted away from an installation path of an armature into a brush holder and is urged against a side wall of the brush holder by an urging means in a direction perpendicular to a sliding direction of the brush, which substantially coincides with a radial direction of a commutator of the armature, so that the brush is temporarily held in the first position by a static frictional force generated between the side wall of the brush holder and the brush. This static frictional force is greater than an urging force of the urging means applied to the brush in the sliding direction. Then, the armature, which includes the commutator, is installed in a predetermined position within a housing of the motor along the installation pass of the armature while the brush is temporarily held in the first position. Next, the brush held in the first position is released by applying an external force to the brush in the sliding direction to overcome the static frictional force, so that the brush is slid to a second position, in which the brush is urged against the commutator in the sliding direction, by an urging force of the urging means that is greater than a frictional force between the side wall of the brush holder and the brush.
To achieve the objectives of the present invention, there is also provided a brush holding device for a motor that includes an armature having a commutator. The brush holding device includes a brush, a brush holder and an urging means for urging the brush and also for selectively positioning the brush between a first position and a second position. The brush holder includes a side wall. Furthermore, the brush holder slidably receives the brush such that the brush is slidable in a sliding direction, which substantially coincides with a radial direction of the commutator of the motor. In the first position, the brush is retracted away from the commutator into the brush holder and is urged against the side wall of the brush holder by the urging means in a direction perpendicular to the sliding direction, so that the brush is temporarily held in the first position by a static frictional force generated between the side wall of the brush holder and the brush. This static frictional force is greater than an urging force of the urging means applied to the brush in the sliding direction. In the second position, the brush is urged against the commutator by the urging means in the sliding direction.
To achieve the objective of the present invention, there is also provided a motor including a motor assembly and a speed reducing gear assembly. The motor assembly includes a motor housing, an armature and a rotatable shaft. The motor housing has an open end. The armature is at least partially received in the motor housing and includes a commutator. The rotatable shaft extends along the axis of the armature and is rotated integrally with the armature. The speed reducing gear assembly includes a gear housing, a worm and at least one above described brush holding device. The gear housing has an open end. The worm is received in the gear housing and is integrally formed with the rotatable shaft. Each brush holder is spaced apart from the open end of the gear housing to be completely received within the gear housing.